Continuous culture is a means whereby organisms can be grown at rates approaching those occurring naturally. Moreover, the effect of adding transient excesses of various nutrients to the culture vessel ('pulsing') simulates the effect of dietary challenge on dental plaque organisms. Mixed cultures of Streptococcus mutans T8 and Streptococcus milleri B448 were grown glucose-limited in a chemically defined medium under an atmosphere of 5 per cent carbon dioxide in nitrogen, at a dilution rate of D = 0.1 h-1 and controlled pH of 7.0. The level of arginine in the medium reservoir was adjusted so that Strep. milleri predominated over Strep. mutans in a stable coexistence. After equilibration, the culture vessel was pulsed with various carbohydrates to a final concentration of 5 x 10(-2)mol/L. Samples were then taken at regular intervals and differential viable counts of Strep. mutans and Strep. milleri were done on mitissalivarius agar. Results demonstrated that pulsing with glucose, fructose, 'coupling sugar', lactose, xylose and sorbitol gave Strep. mutans a clear ecological advantage. In direct contrast, pulsing with xylitol resulted in a marked antimicrobial effect on Strep. mutans while Strep. milleri was essentially unaffected. This supports recent findings by other workers that uptake of this pentitol by Strep. mutans in batch culture sets up a 'futile cycle', leading to depressed growth or even cell death.